The decision by Justice Jennifer Schecter of State Supreme Court in Manhattan paves the way for lawyers to seek depositions from several women who accused Mr. Trump of sexual harassment before he was elected and to subpoena Trump campaign records related to his female accusers. Her lawsuit followed on the heels of a suit filed by Stormy Daniels, an adult film star who said she was paid to keep quiet about her own alleged affair with Trump long before he took office.
The decision raises the possibility of the President of the United States being forced to answer questions about his behavior toward women, and being required to hand over documents from his past.
The lawsuit comes a month after the New Yorker reported on the alleged affair and a move by American Media to pay Ms McDougal for exclusive rights to her story, which it never published.
PHOTO: Summer Zervos, right, leaves New York State Supreme Court with attorney Gloria Allred after a hearing on the defamation case against President Donald Trump in Manhattan on December 5, 2017.
Trump's attorney Marc Kasowitz meanwhile announced plans to immediately appeal and seek a stay of the ruling.
"In holding that the doctrine of separation of powers did not mandate a stay of all private actions against the president, the court flatly rejected that 'interactions between the judicial branch and the executive, even quite burdensome interactions, necessarily rise to the level of constitutionally forbidden impairment of the executive's ability to perform its constitutionally mandated functions", Schechter said of the 1997 case.
"Nothing in the supremacy clause of the United States Constitution even suggests that the president can not be called to account before a state court for wrongful conduct that bears no relationship to any executive responsibility", Schechter said.More news: Experts stress need to report TB cases to eradicate it
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Speaking of her client's victory, Allred said: "I am very happy".
Former Playboy model Karen McDougal claims she had an affair with Trump in 2006.
Schecter wrote that Trump's comments calling Zervos a liar "cannot be characterised simply as opinion, heated rhetoric or hyperbole".
Zervos is seeking an apology from Trump and "at least" US$2914 ($3793) in damages.
She sued in January a year ago. At a presidential debate, Trump denied ever doing the things he described in the recording, which some of his accusers said prompted them to come forward.
Zervos, a 2006 Apprentice contestant repped by attorney Gloria Allred, claims Trump sexually assaulted her twice in 2007.
Trump had argued that he was entitled to presidential immunity while serving as commander-in-chief and did not have to respond to the suit filed by Summer Zervos.